Beirut: The Syrian army is poised to slice rebel-held eastern Ghouta in two as forces advancing from the east link up with troops at the enclave's western edge, a pro-Damascus military commander said on Thursday.

That put the zone effectively under Syrian government control as the remaining strip of territory was within weapons range.

The government, backed in the war by Russia and Iran, is seeking to crush the last major rebel enclave near Damascus in a ferocious campaign that the Syrian Observatory for Human Rights says has killed 893 civilians in the last 18 days, including 91 on Wednesday.

The pro-Damascus commander, who spoke on condition of anonymity because he was not authorised to talk to the media, confirmed a report by the Observatory late on Wednesday that the enclave had effectively been sliced in two.

But Wael Alwan, the Istanbul-based spokesman for Failaq al-Rahman, one of the main rebel groups in eastern Ghouta, denied that the territory had been cut in half. "No" he said in a text message when asked if the report was correct.

An aid convoy that intended to go to Ghouta later on Thursday was postponed, the International Committee of the Red Cross (ICRC) said.

The United Nations says 400,000 people are trapped in the towns and villages of eastern Ghouta. They have been under government siege for years and were already running out of food and medicine before the assault. Many civilians have fled from the frontlines into Douma, a town in the enclave.

Russia, President Bashar al-Assad's most powerful ally, has offered rebels safe passage out with their families and personal weapons. The proposal echoes previous agreements under which insurgents, in the face of military defeat, were permitted to withdraw to opposition-held areas along the Turkish border.

Russia's defence ministry said on Wednesday some rebels wanted to accept the proposal to evacuate. So far rebels have dismissed it in public and vowed to fight on.

The pro-Damascus commander said the remaining sliver of territory linking the northern and southern parts of the enclave was just 1 km wide and within firing range of government forces. In military terms, that meant the territory had been bisected, the commander told Reuters.

Defeat in eastern Ghouta would mark the worst setback for the anti-Assad rebellion since the opposition was driven from eastern Aleppo in late 2016 after a similar campaign of siege, bombing, ground assaults and the promise of safe passage out.

Moscow and Damascus say the Ghouta campaign is necessary to halt deadly rebel shelling of the capital.

The U.N. Security Council called on Wednesday for the implementation of a February 24 resolution demanding a 30-day ceasefire across Syria and it voiced concern about the country's humanitarian plight.

An aid convoy entered Douma on Monday but it was stripped of some of its medical supplies by the government and unable to completely unload as fighting raged on. The United Nations had hoped to deliver the outstanding aid on Thursday.

"Today's convoy is postponed," ICRC spokeswoman Iolanda Jaquemet told Reuters. The United Nations had asked the government to commit to a ceasefire on Thursday to let in more aid.